Method of cutting gears



July 28, 1942. R. s DRUMMOND METHOD oFfcUT'rIriG GEARS Filed July 30, 19:2

4 Sheets-Sheet, l

3mm Robert S. Frzufiwan Z M928, 1942. H R. s. DRUMMOND r 2,291,537

I METHQD CUTTING GEARS Filed July so} 1932 4 Sheets-Sheet 2 Jul 23, 1942.

R. s. DRUMMOND METHOD F CUTTING emns Filed July so, 1932 4 shet -gxef a considerable time.

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PstenteJ JuIyzs, 1942 Ji a UNITED STATES PATENT. OFFICE METHOD OF CUTTING GEARS Robert S. Drummond, Detroit, Mich. Application July so, 1932, Serial N... 626,768 1 8 Claims. (01. 9 -1.6)

The invention relates to the finishing of gears which have been roughly fashioned by any suitable method such for instance as by hobbing. It is the object of the invention to rapidly remove sufficient stock to correct all errors in the original form and to obtain a high degree of accuracy as to tooth contour, helical angle, circular pitch, indexing, etc. In the present state of the art it is usual to make correction in gears after the heat treatment of the same by grinding or lapping. Where, however, the gear is first roughly formed, a considerable amount of stock must be removed to make proper correction, which in the case of either grinding or lapping consumes I have therefore devised a method and apparatus which is particularly adapted for correcting the form of a rough cut gear prior to the heat treatment of the same so that the amount of grinding or lapping necessary after heat treatment is reduced to the Broadly stated, my improvement consists in running .the rough gear in mesh with a finishing plane and having an angular difierence between' the limits of three degrees and thirty. degrees.

The tool is of a form similar to that of a lap or intermeshing gear, but is provided with one or,

more grooves which interrupt the continuity of; surface longitudinally of each tooth. 'I'his'has .the effect of causing a cutting of the surface as hereinafter described and as shown in the accompanying drawings in which:

Figure 1 is a perspective view of the machine showing the finishing tool' in engagement'with the gear to be finished;

Figure 2 is a vertical central section through the machine;

Figure 3 is a sectional plan view;

Figure 4 is a perspective view showing a portion of the tool and a gear to be finished in operative relation thereto; I

Figure 5 is a diagram illustrating the operation of the machine and the electric timing mechanism therefor;

Figures 6 and 6a are respectively a plan view and elevation of one tooth in a modified con struction of tool;

Figures 7, 7a, 8, 8a, 9, 9a, 10, 10a, and 11, no show other modifications;

.- Figure 12 is a plan view of another modifica- I like that shown in Figure 4 except that the helixangle is inclined in the opposite direction;

Figure 16 is a cross section taken on the line l6-l6 of Figure 15;- g

Figure 17 is a top plan view of a modified form of finishing tool;

Figure 18 is a diagrammatic view representing the developed pitch plane of an intermeshing gear and cutter illustrating the cutting action'of the cutter.

The geometrical principle involved in my im-'. proved method and apparatus is as follows: when two cylinders 'are placed in peripheral contact with their axes at an angle to each other, they will theoretically form only a point contact which during rotation of the cylinders about their axes will generate lines upon the cylindrical surface. Practically, the actual contact extends over an area, the dimension of which varies according to, first, pressure of contact and second, the angular difference between the axes of the cylinders, being greatest in width when the angular difference is small and diminishing in width as the angular difference increases. During rotation of the cylinders about their axes, this contact area will generate a band about each cylinder. The extension of the area of contact is due to a deformation in the cylinders which in case both cylinders are formed of material of the same hardness will be equal in each. If, however, one cylinder is of harder material than the other, the deformation or indentation will be greatest in the latter. Also, if the surface of the harder cylinder is partly cut away, the remaining portion will bite into the softer cylinder and during rotation will produce a cutting action. i

Gear teeth are modified-cylindrical surfaces and where intermeshing gears with conjugate teeth have their axes non-parallel and in no common plane, the contact-between the teeth Based upon the geometrical principles just referred to, I have devised a cutting tool, the body of which is in the form of a gear having teeth conjugate to the gear to be cut. The helical angle of these teeth is, however, such that the axis of the tool will be non-parallel to the axis of the work gear, will have no common plane therewith and will be at an angle thereto within the limits .of 3 degrees and thirty degrees. 33; then forming a groove or slot in the surface of the teeth ,of said tool extending in a direction transverse to the mesh with the work under a predetermined load, a cutting of the work is effected. If the work gear is also slowly reciprocated' axially thereof, this cut will be spread over the entire surface of the teeth and will correct the'form, thereof.

Machine a The machine for performing the work may be I of any suitable construction such as used for the lapping of gears, but comprises essentially an arbor A for the gear to be finished, an arbor B for the finishing tool atfillstable in spacing and in angular relation to each other so that the work may be placed in intermeshing relation with the tool. As shown in Figures 1 and .5 the medium of a step-down transmission D, shaft D, pulley D, belt E, and pulley E. The arbor A is mounted between head and tail stocks F and G on a carriage H which is slidably mounted on ways I on a head I vertically adjustable on a column J. This column is supported on arcuate.

ways K on the bed L on which latter is mounted the arbor B and drive mechanism therefor. Thus the-arbor A may be angularly adjusted with respect to the arbor B by adjustment of the column J around the arcuate ways K. The spacing between the arbors can be adjusted by raising or I lowering the head I on the column J and the arbor A may be axially reciprocated by movement of the carriage H on the ways I. As shown, the 'mechanism'for accomplishing this reciprocation consists of a further step-down gearing M which I "actuates a crank M adjustable in throw which in turn is connected by a link N to a vertically extending rock shaft N. Splined on this rock shaft N' is a gear wheel N for actuating a rack N? on the carriage H. Thus simultaneously with the rotation of the arbors A and B. a slow reciprocating movement isimparted to the arbor A in' an axial direction bythe movement of the carriage H. The amount of movement is determined by the adjustment of the throw of the crank M' and the timing is such that for a rotational speed such as 400 R. P. M.the reciprocadirection of progressive rubbing contact with the teeth of the work, and by running this tool in the arrow V (i. e. down in the drawings) and the arbor B is driven from a motor through the tion of the carriage is only 4 inches per minute.

The reciprocation should be proportioned to the diameter of the work to provide a selective rate of.feed for each revolution of the work. The

amount of reciprocation of the work is just sufflcient to cause the cutting edges of the finishing tool to traverse the entire width of the work gear without permitting the work gear to pass out of contact with the guiding surfaces of the finishing tool on opposite sides of the cutting edges.

The machine is also preferably provided with automatic reversing mechanism by which, after the work has completed one stroke of the reciprocation, it is reversed and fed in the opposite direction. It is also preferable to reverse the direction of rotation of the work gear and finishing tool when the feed is reversed. Any suitable reversing mechanism may be employed but, as diagrammatically shown in Figure 5, there is a reversing electric switch 0 actuated through tim. ing gears O which causes the reversal of the electric motor C after a predetermined number of revolutions thereof.

In order to obtain the most effective cutting action the direction ofthe cross feeding movement of the gear with respect to the cutter should be preferably in a certain predetermined relationship with the rotation of the cutter. This can intermeshing bottom portion of the gear'Q will have the motion represented by the arrow V It will be noted that this motion of the gear Q represented by the arrow V crosses the cutter P from left to rightfTherefore the correct cross feeding motion of the gear Q should also be from left to right as indicated by the arrow V Thus in the timing of the reciprocation of the gear arbor A and the rotation of the cutter shaft B it is always preferable to obtain the relative motions indicated above. When, as previously stated, the cross feeding movement of thegear is reversed it is preferable to simultaneously reverse the rotation of the cutter in order to obtain the best cutting effect. I

Finishing tool As has been stated, the finishing tool P is in the form of a mating gear having conjugate teeth of such a helix angle in relation to the helix angle of the gear to be finished thatthe axes of the arbors -B and A on which said gears are respectively mounted are at a limited angle to each other. The angular difference is between the limits of three degrees and thirty degrees but 1 preferably is ten degrees. To produce the cutting action the teeth of the tool are grooved or slotted so as to interrupt the continuity thereof and the direction of this groove or slot is transverse to the direction of progressive rubbing contact between the teeth of the tool and those of the gear to be finished. As shown in Figure 4, the interruption of the teeth is formed by a central groove P parallel to the plane of rotation ofthe gear and extending the full depth of the teeth. The

sides of this groove flare in an outward-direction and the angle of this flare is preferablythe same as the difierence in helix angle between the arbors A} and B, such for instance asten degrees.

In setting the machine for operation the rough cut gear Q to be finished is mounted on the arbor A and the tool P upon the arbor B. The tool is also axially adjusted so that the groove P will extend on opposite sides of the theoretical point of contact between the teeth of the tool and those of the work gear. The head I is then adjusted downward to crowd the gear Q against the tool gear P with a predetermined amount of pressure. The position of the head is noted, the head is raised again, traversed to the edge of the cutting tool, and the head is brought back to the same position as previously noted. The motor C is then started in operation which will rotate the two gears P and Q in mesh with each other, while a slow axial reciprocation is imparted to the gear and cutter substantially at the zone of contact.-

. In this figure the arc aa represents the original curved surface of the tooth in the work gear and the are 27-!) represents the original curved surface of the tooth in the finishing tool P.

2,291,587 These arcs are shown as overlapping each other and the groove P in the tool P extends upon Opposite sides of the center of this overlap. Assuming that the two members Q and P are crowded in contact with each other, this would.

have the eifect of flattening the surface of each over an area on each side of the groove P as indicated by the straight lines cc and c'-c'. However, in the central portion of the member Q which is opposite the groove P .the surface will extend outward to lie in the arcc-d. If now the members P and Q are moved relatively in the direction of the arrows, the sharp edge d atone side of the groove P' will bite into the work, cutting oil the portion e which extends into the groove so as to form a surface represented by the arc indicated at d. It will be understood that the dimensions in this diagram are greatly exag- 7 the blanks R and R2 and lie in a continuation" edges are of the exact contour of the teeth in of the surface of said teeth, but because of the difference in helix angle grooves are formed on of the surface, in which case greater cuts will be obtained. Conversely the teeth can be placed a certain distance below the surface continuation and a cutting action (somewhat diminished) will still be obtained. One of the advantages of the gerated and the actual cut is only a matter of thousandths of an inch, or fractions of a thousandth.

'Where there are errors in form-in the work gear, such for instance as eccentricity, the increased pressure between the tool and work in the portions of. longer radiuswill produce a cor- -forms according to the character of the work to be performed. 11', for instance, it is desired to correct the tooth contour, this may be accomplished by forming the groove of varying widths according to the nature of the correction 'de-' sired. Some of the various modifications are illustrated ,inFigures 6 to 11 in all of which the teeth of the tool are grooved but with varying angles-to the sides, these sides flaring outward in Figures 6 to 8 and flaring inward or towards the root of the tooth in Figures 9 to 11.

Another form of finishing tool is illustrated in Figure 14 where the groove P in each tooth extends at an oblique angle and the finishing tool has helical teeth. 'This construction will maintool illustrated in Figure 12 resides'in the fact 'that the guiding portions R and, R may be made of different metal than the cutting disks B and R If desired, the guiding portions R and R may be made of cast iron or other suitable heating metal, while-the cutting disks R and R are of the next disk is backed off or otherwise re-.

lieved as indicated at T on one side of the helical tooth and at T on the opposite side.

This results in the formation of a series of.

lieved, leaving unrelieved portions T which serve as guides for the teeth of the work engaging therewith. It will be observed that each of the cutting edges T is arranged between two unrelieved guiding portionsT".

Instead of producing the contact pressure between the tool and the, work by crowding or cramping the one in relation to the other, it may be formed by loading or resisting the rotationof the work gear which is driven by the tool. This modification will correct errors in tooth form and'spiral angle butthe cramping action is better suited for correcting the work as to index and eccentricity as well as spiral angle and tooth form.

With the several forms of gear finishing tools heretofore described there are certain charactertain the bearings of the tool on the work uniformly spaced on opposite sides of the theoreti- Two of these blanks R, R which form the opposite sides of the tool, are outwith the teeth of the same contour and helix angle. Between these are arranged two other thin disk blanks R and R which are cut to the same tooth contour but -with a slight difference in helix angle and a spacer disk R between the same. This results, when the several blanks are assembled, in forming cutting edges between abuttingfaces of the blanks R and R which project beyond each other on opposite sides of the tooth so that the edge of the blank R on one side will form the cutter and the edge of the blank R. on the opposite side of the tooth will form the cutter. These respective shafts.

istics in common. Also, the method of cutting gears with the various forms of tools is distinctly different from the methods heretofore employed for, cutting gears. f

One of'the characteristic features of my in-- vention is that the work gear is finished with a rotary cutting tool. Another feature of the inventionresides in the fact that the method does' not require the timed rotation of thework and the cutter by means of gearing connecting the It -is to be noted that in this new method the relative-rotation of the work and the cutter depends entirely upon the intermeshing of the work gear and cutter gear, and the accuracy of the work is predicated upon the accuracy of the tool only; as distinguished from the usual forms of gear cutting wherein the timing of the gearcutting machine is a major factor.

My invention has the further feature that the is shown above the cutter P, the axes being arranged at an angle between three degrees and crossed axes slide.

thirty degrees. The gash I in the cutter dividesone toothinto two portions P and P and the next adjacent tooth into similar portions P and P. The tooth Q of the gear Q isIepresented as intermeshing between-the two adjacent cutter teeth. If this tooth is caused to move in the direction U with respect to the cutter teeth, the edge X formed by the side of the gear tooth P and the gash P becomes the cutting edge, the angle of which is less than ninety degrees. While the cutting action takes place at this edge X, it will be observed that the gear tooth Q bears. against the tooth faces Y, Y, Y and Y, thus,

' accurately positioning the gear tooth Q with respect to the tooth profiles of the cutter.

When the gear tooth Q is moved in the opposite direction from the arrow U it will 'be observed that I the diagonally opposite edge X becomes the cutting edge. The tooth, however, bears against all of the side faces Y, Y,- Y and Y. It will be observed therefore that the cutting action of my improved rotary cutter is rendered extremely accurate because of the fact that the tooth being cut is guided on opposite sides of the cutting edge by the accurately formed guiding tooth faces Y,

It is also to be observed that the accuracy of the cutting action is due to the numerous varyingcontacts of each gear tooth with the teeth of the rotary cutter. This renders the index obtained from the finishing tool closer than the index quality of the tool itself. It also follows that the eccentricity and profile are more accurate because of the number of contacts made by the rotary tool with difierent points on the Profile I of the teeth and on different teeth. The finishing tool also establishes the spiral angle by its setting' and profile.

It is well-known that the specific sliding contact between the surfaces of the teeth of a pair of meshing involute gears operating on parallel axes is variable. When the meshing teeth contact each other at the pitch line the sliding is, zero and at this point rolling contact takes place. The specific sliding is a maximum at the tops and bottoms of the engaged teeth. This variable sliding action is referred to as involute slide.

Due to the fact that according to the present invention, the gear and tool are operated in nonparallel intersecting planes, a separate sliding contact is introduced between the surfaces of the engaged teeth. This sliding contact is a function of the angle between the axes of the gear and cutter and is substantially uniform from top to bottom of the engaging teeth. It operates longitudinally, that is, from end to end, of the teeth.

Actual instantaneous sliding contact between any engaging point on the surface of meshing is termed the involute slide"; and the other of which operates in a direction generally longitudinal of the teeth, and .which is referred to as' The various cutters illustrated herein are'provided with one or more cutting edges extending generally in the plane of roll of the cutter. The cutting edge or cutting edges thus positioned extend generally parallel to that component of total '0 relative sliding'motion on the engaged teeth which has been described as involute slide. The cutting edges therefore extend generally perpendicular to that component of the relative sliding mo tion which extends longitudinally of the meshing 7 teeth and which has been referred to as (crossed axes slide." The result of this is that the cutting action resulting from meshed rotation of the gear and tool as described herein is substantially. uniform from top to bottom of the teeth of the gear and insures a very accurately finished gear tooth surface. As previously stated, it is desirable to have the angular difference between the axis of the work and the axis of the rotary cutting tool between the limits of three degrees and thirty degrees. By having even a slight angular difference between the axis of the work and the tool (such for example as ten degrees) free cutting is obtained with light pressures and the tool also acts as a guide for the work with good supporting contact. If the angle is increased substantially beyond thirty degrees, the bearing between the tool and the work decreases and the guiding support is no longer ample to give extreme accuracy to the gear teeth out by the tool.

In the above description it will be observed that I have described a new method of cutting gears and I have also described a rotary cutting tool. of novel form. It is to be understood that my invention'is not to be construed as limited to the precise details of construction hereinbefore set forth except in the light of the claims appended hereto.

What I claim as my invention is:

l. The method of truing gears which consists in running a rough cutgear in mesh and in pressure contact with a tool having the form of a true mating gearwith conjugate teeth, the axes of said gears having no common plane, being non-intersecting and at a limited angle to each other, said tool having'the surface of its teeth interrupted to form a narrow groove and adjacent cutting edge extending generally up and down the teeth transverse to the direction of tion parallel to the axis of the said gear and in timed relation to the rotation of the said gear.

2. The method of truing gears which consists in running a rough cut gear in mesh and in pressure contact with a tool in the form of a true mating gearwith conjugate teeth, theaxes of said gears having no common plane, beingnonintersecting and at a limited angle to each other, the surface of the teeth of said tool being interrupted to form a narrow groove extending generally up and down the teeth transverse to the direction of the progressive rubbing contact with the gear to be trued and having contact bearings therewith on opposite sides of said groove whereby one edge of said groove will bite into the work and form a cut of' predetermined depth limited by the bearings on opposite sides thereof and providing a slow relative feed between the said gear and tool in a direction parallel to the axis of the said gear and in timed relation to the rotation of the said gear.

3. The method of finishing the teeth of a gear member which comprises meshing it with a circular gearlik'e cutter member having cutting edges provided on the teeth thereof generally parallel to the plane of roll-of said cutter member, 'with the axes of said gear member and cutter member crossed at an angle of less than 30; rolling said members in mesh by driving one of said members directly and thereby driving the other of said members through their meshed relation, and providing a slow relative feed between said members in timed relation to the rotation thereof and in a plane parallel to the axesof both of said members to distribute the cutting action of said cutter member from end to end of the teeth of said gear member.

said teeth. which varies from a minimum at the pitch line to a maximum at the tops and bottoms v of said teeth, and crossed axes slide resulting ..from rotation of said members in intersecting 4. The method of finishing the teeth of a gear at an angle of less than rolling said members,

in mesh by driving one of said members directly and thereby driving the other of said members through their meshed relation; and providing a slow relative feed between said members in timed relation to the rotation thereof and in a plane parallel to the axes of both of; said members to distribute the cutting action of said cutter member from end to end of the teeth of said gear member.

5. The method of finishing the teeth of a gear member which comprises meshing it with a circular gearlike cutter member having cutting edges provided -on the teeth thereof generally parallel to the plane of roll of said cutter member, with the axes of said gear member and cutter member crossed at an angle of less than 30; rolling said members in mesh by driving one of said members directly and thereby driving the other of said members through their meshed relation; and providing a-slow relative feed between said members in timed relation to the rotation thereof and 7 parallel to the axis of said gear member to distribute the cutting action of said cutter member from end to end of the teeth of said gear member.

6 The method of finishing the teeth of a gear 1 member which comprises meshing it'with a circu largearlike cutter member having cutting edges provided on the teeth thereof generally parallel to the plane of roll of said cutter member, with planes and is substantiallyuniform from top tobottom of said teeth and generally perpendicular to the said involute slide, the said cutting edges being generally parallel to the component of total sliding due to involute slide; and providing a slow relative feed between said members-in timed relation to the rotation thereof and in a plane the axes of saidgear member and cutter member crossed at an angle of less than 30"; rolling said members in mesh by driving one .of said members directly and thereby driving the other of said members through their meshed relation; thereby effecting a relative sliding between the surfaces of the engaged teeth which is the reparallel' to the axes of both. of said members to distribute the cutting action of said cutter member from end to end of, the teeth of said gear member. v 7. The" method of finishing the teeth of a gear member which comprises meshing the gear member with a circular gear like cutter member having cutting edges provided on the teeth thereof v generally parallel to the plane of roll of the cutter member, with the axes of-the gear member and cutter member'crossed at an angle of less than 30 degrees; rotating the members in mesh to give the cutting'ed'ges of the cutter member a component of lateral motion relative to the teeth of the gear member; and effecting a slow relative feed between the members in timed relation to the rotation thereof to give the cutting edges of the cutter member a second component of lateral motion relative to the teeth of the gear member in the same direction as the, first mentioned component of lateral motion.

8. The method of finishing the teeth of a gear member which comprises meshing the gear mem- "ber with a circular gear-like cutter member having cutting edges provided on the teeth thereof generally parallelvto the plane of roll of the cutter member, with the axes of the gear member andcutter member crossed atfian angle of less than 30 degrees; rotating the embers in mesh to give the cutting edges of th cutter member a component of lateral motion relative to the teeth of the. gear member; effecting a -slow relative feed between the members in timed relation to the rotation thereof to give the cutting edges of the cutter member a second component of lateral motion relative to the teeth of the gear member in the same direction as the first mentioned comsultant of involute slide generally up and down ponent of lateral motion; and reversing the rotation of themembers' in mesh and at the same time reversing the slow relative feed between the members.

ROBERT S. DRUMMOND. 

